1. Field of the Invention
The present invention relates to a fusing device incorporated into an image forming apparatus, used for fusing a toner image formed on a recording medium.
2. Description of the Prior Art
Concerning a conventional method for fusing a toner image formed on a recording sheet, there is provided a nip portion between a pair of rollers, wherein one is a heating roller and the other is a pressurizing roller coming into pressure contact with the heating roller, and the recording sheet on which an unfused toner image is formed is made to pass through the nip portion, so that the toner image can be thermally fused on the recording sheet. Recently, there is a tendency that the processing speed of an image forming apparatus is remarkably increased. Also, there is a tendency that the color image forming apparatus has been widely used. Especially when a multi-color image is fused on a recording sheet, it is necessary to ensure a stable coloring property of the fused color image.
Ensuring a stable coloring property of the fused color image can be accomplished when the masking of a lower toner layer conducted by an upper toner layer is removed. The masking of the lower toner layer conducted by the upper toner layer can be substantially removed as follows. The toner layer is heated and fused by a fusing device. At the same time, the toner layer is given a high pressure by the fusing device, so that voids formed on the toner layer can be reduced, and the toner layer is compressed to be a thin layer. As a result, the upper toner layer color and the lower toner layer color are appropriately mixed with each other. Therefore, it is possible to accomplish an excellent coloring property. Even if, a small toner grain boundary exists at this time, the coloring property is not affected. In this connection, since voids on the toner layer are reduced, the toner layer surface is made to be flat, so that the fused toner image becomes glossy. It is possible to change the glossiness in a range from a value not less than 5 to a value smaller than 20 when the heating and fusing time of the toner layer is changed and also the compressing time is changed while the coloring property is maintained high.
In the case where a multi-color image is fused on a transparent sheet, it is necessary to provide a high transparence. It is possible to realize a high transparence by preventing light from diffusing on the toner layer.
In order to prevent light from diffusing on the toner layer, it is necessary to heat, fuse and compress the toner layer while a longer time than that required for fusing the multi-color image on a recording sheet is taken, so that the voids and the grain boundary on the toner image, which are the causes of diffusion of light, can be removed and so that the toner layer surface can be made as flat as a mirror surface. In this connection, the glossiness can be changed by a value not less than 20 while a high transparence is maintained.
In the case where a monochromatic image is fused on a recording sheet or alternatively a black image is fused on a transparent sheet, it is sufficient that the adhesive strength of the toner layer onto a recording sheet is ensured. Accordingly, in this case, a period of time required for heating, fusing and compressing the toner layer may be shorter than that required for fusing a multi-color image on a transparent sheet described above. In this case, since the toner image contains a number of voids, there are irregularities on the surface, and the glossiness can be changed by a value smaller than 5 while the adhesive strength is ensured.
Accordingly, when the multi-color image is fused, it is necessary to heat, fuse and compress the toner layer while a longer period of time than that of a case of fusing a monochromatic image is taken. Therefore, in order to fuse a multi-color image by the conventional fusing device composed of a pair of rollers, it is necessary to decrease the passing speed of a recording sheet, or alternatively it is necessary to increase a width of the nip portion so that a period of time in which the toner layer on a recording sheet passes through the nip portion can be extended.
However, the method of decreasing the passing speed of a recording sheet is against the tendency of increasing the processing speed. Also, the method of increasing the width of the nip portion by extending a roller diameter is disadvantageous in that the size of the image forming apparatus can not be decreased. Also, the method of increasing a contact force of the pressurizing roller with the heating roller is disadvantageous in that the nip portion width can not be extended so long and the toner layer of a multi-color image can not be sufficiently heated and fused, and further the durability of rollers is deteriorated and wrinkles are caused on a recording sheet.
In order to solve the above problems, Japanese Patent Publication No. Sho. 55-29822 discloses the following method. An endless belt is trained round two pressurizing rollers, and the endless belt is made to come into pressure contact with one heating roller, so that a longer nip width can be provided. Also, the following method is disclosed. Two pressurizing rollers are made to come into pressure contact with one heating roller, and a stationary guide mechanism is arranged between the two pressurizing rollers while the stationary guide mechanism is separate from the heating roller, so that a recording sheet can be contacted with the heating roller. Due to the above arrangement, the number of the nip portions is increased, so that a period of time for heating, fusing and compressing the toner layer can be increased.
However, according to the method disclosed in Japanese Publication No. Sho. 55-29822 in which the nip portion width is extended by making the endless belt to come into pressure contact with the heating rollers, a contact force of the pressurizing roller with the heating roller via the endless belt is set at a low value so that wrinkles or elongation can not be caused on the endless belt. Consequently, an intensity of the contact force of the endless belt with the heating roller is maintained low. For this reason, the toner layer is not sufficiently compressed. Accordingly, in the case of fusing a multi-color image, the masking of the lower toner layer conducted by the upper toner layer can not be sufficiently removed, and the coloring property is deteriorated.
When toner (sharply melted toner), the fusing point of which is lower than that of common toner, is used to solve the above problem, toner tends to adhere onto a restricting blade and a cleaning blade of the developing unit of the electrophotographic apparatus. As a result, defective images tend to occur, and further cleaning can not be appropriately to carried out, so that the durability is deteriorated. Furthermore, it is difficult to rotate the endless belt at high speed without vibration and slippage onto one side. Consequently, it is difficult to highly increase the processing speed of the image forming apparatus.
In the arrangement disclosed in Japanese Patent Publication No. Sho. 55-29822 in which two pressurizing rollers are made to come into pressure contact with one heating roller, a stationary guide mechanism is arranged between the two pressurizing rollers while being separate from the heating roller, so that the following problems occurred. That is, the fusing device comprises a first roller (a heating roller as a heating member) for heating a recording medium or a recording sheet of paper, a second roller (a pressurizing roller as a pressurizing member) pressed against the first roller to form a first nip portion at a two-dimensional area of contact therebetween, and a third roller (a pressurizing roller as a pressurizing member) pressed against the first roller to form a second nip portion at a two-dimensional area of contact therebetween. The fusing device heats, fuses and permanently fixes a toner powder image formed on the recording sheet by passing the recording sheet through the first nip portion and the second nip portion. In the roller arrangement of the fusing device, the second and third rollers are turned by the first roller with the aid of frictions each between the peripheral surfaces of the adjacent rollers.
In the fusing device thus constructed, the outer peripheral surface of the second roller is softer than of the first roller. The or surface of the third roller is harder than of the first roller. A contact pressure in the nip portion causes a surface expansion therein, and a transfer of a torque through the nip develops a shearing stress. When the surface of the roller moves out of the nip, the surface contracts, and a peripheral speed of the roller decreases. A peripheral speed of one roller whose surface is more deformable is lower than of the other whose surface is less deformable. In the prior art, the second roller is slower than the third roller when these rollers turn.
In the three-roller fusing device, when the roller diameter or the recording sheet thickness is varied, a quantity of deformation of the roller varies, and hence a distance from the roller center to the roller surface also varies. This possibly causes a variation of the roller transporting speed.
Also when a coating amount of oil and/or roller temperature varies transporting force is varied. Sometimes, this entails an, and a frictional force on the roller surface varies, a substrate instable relative shift between the roller surface and the copy sheet.
In the three-roller fusing device disclosed, the second and third rollers serve as follower rollers. Because of this, the fusing device suffers from the following disadvantage. The transport speed of the third roller is varied by 1) the thickness of the recording medium, 2) the surface conditions of the second and third rollers, for example, oil coating conditions, and 3) temperature of the first to third rollers. The variation of the transport speed of the third roller makes it difficult to keep the peripheral speed of the third roller higher than of the second roller.
When the transport speed is instable, the recording medium is sometimes slackened between the second roller and the third roller. In this case, the recording medium is detached from the first roller between the second roller and the third roller, and there will not be enough heat transfer to deteriorate the fusing/fixing performance. Further, if the slackening of the recording medium is further increased, a tension of the recording medium in the roller axis direction is lost, so that a gentle undulation of the recording medium tends to occur in the roller axis direction. When the roll pair receives the undulated recording medium in the second nip portion, the recording medium is wrinkled. When the wrinkles of the recording medium become heavy, a torque for the paper transport rapidly increases. When the transport torque exceeds the torque by the drive source, the paper transporting operation stops, causing paper jams.
Further, it is difficult to remove a recording sheet in the occurrence of paper jams in which the fusing device is jammed by the recording sheet, since the stationary guide obstructs the operation of jam clearance.
On the other hand, the subjective judgment of beauty and preference on an electrophotographic image is greatly affected by a combination of the glossiness of a toner image with the surface property of a recording sheet. There are various types of recording sheets. Examples of usable recording sheets are: rough paper such as reclaimed paper and bond paper, the surface of which is rough, and the glossiness of which is low; plain paper, the glossiness of which is intermediate; and smooth paper, the surface of which is smooth and glossy. Inventors made investigation into the glossiness of images and recording sheets and the subjective values of evaluation while the fusing conditions were variously changed and images of various glossiness were printed and fused on recording sheets of various surface roughness. As a result of the investigation, it can be concluded that the smaller the difference between the glossiness of images and the glossiness of recording sheets which are the background of the images, the higher the subjective values of evaluation. In other words, there is a tendency that the subjective values are high when the glossiness of an image is the same as the glossiness of the background. That is, a mat surface, the glossiness of which is low (the glossiness is lower than 5), is suitable for rough paper. A gloss surface, the glossiness of which is high (the glossiness is not lower than 20), is suitable for smooth paper. The intermediate glossiness (not lower than 5, and lower than 20) is suitable for paper of the intermediate surface roughness.
However, in some cases, images of high glossiness or images of low glossiness are preferred irrespective of the surface roughness of recording sheets. For the above reasons, in order to meet the demands of all users, it is preferable that the glossiness can be arbitrarily selected in accordance with their preference.
In the conventional fusing system in which periods of time to heat, fuse and compress a toner layer are set at constant values, in order to fuse monochromatic images or multi-color images on various recording sheets, it is necessary that the periods of time to heat, fuse and compress a toner layer are set in accordance with the fusing conditions for fusing a multi-color image on a transparent sheet, wherein the periods of time to heat, fuse and compress a toner layer are the longest when a multi-color image is fused on a transparent sheet.
However, the following problems may be encountered in the above conventional fusing system. As described before, the glossiness of an image is increased to a value not lower than 20 at that time. Therefore, especially when rough paper is used as a recording sheet, the glossiness of an image and the glossiness of the background are greatly different from each other, and everybody feels a sense of incongruity. Even when a user likes an image of low glossiness, only an image of high glossiness is provided.
According to the method in which the glossiness is changed when a pushing force of the pressurizing roller to the heating roller is changed, an amount of change in the width of the nip portion is small. Therefore, an amount of change in the period of time of heating, fusing and compressing a toner layer is small. Accordingly, the glossiness can be changed only in a range from the intermediate glossiness to the high glossiness (from 10 to 20). Consequently, everybody feels as sense of incongruity when the toner image is fused on rough paper. Even when a highly accurate pressurizing roller and heating roller are used, they have a small camber and further their diameters fluctuate in the longitudinal direction. Accordingly, the width of the nip portion fluctuates when these rollers are rotated. When a pushing force of the pressurizing roller to the heating roller is reduced, the fluctuation of the width of the nip portion affects the average width of the nip portion, and unevenness of fusing becomes remarkable.
In an arrangement in which a belt is made to come into pressure contact with a heating roller, when the contact width (the nip portion width) of the belt with the heating roller is changed, a period of time to heat, fuse and compress a toner layer is changed. In the above system, in the same manner as that of the method disclosed in Japanese Unexamined Patent Publication No. 55-29822, an intensity of the pushing force of the belt to the heating roller is low. Therefore, it is impossible to sufficiently compress the toner layer. As a result, in the case of fusing a multi-color image, the masking of the lower toner layer conducted by the upper toner layer can not be removed, and the coloring property is deteriorated.